The present application is directed to lighting devices, and more particularly to improved program start ballast circuits for discharge lamps. Electronic ballasts are used to power fluorescent lamps, high-intensity discharge lamps, and the like, and typically include an inverter to generate lamp power. Electronic ballasts may be started using one of several starting techniques, including “instant” start, “rapid” start, and “programmed” start. The instant start technique starts a lamp without preheating a cathode associated therewith, which results in low cost in ballast design but the lamp cathodes can be degraded rapidly due to the violent nature of the starting method. Rapid start ballasts start the ballast and heat the cathode concurrently, resulting in a relatively long start time while mitigating the adverse effects of a cold start on the lamp's cathode. Programmed start ballasts apply a relatively low output voltage initially, which is not high enough to begin gas discharge, while the lamp filaments or cathodes are preheated at a relatively high level for a limited period of time. After the cathodes are preheated, a moderately high voltage is applied to ignite the lamp and the filament heating power is discontinued. Conventional programmed start ballasts open or short a preheat circuit to stop the preheating power (cathode cut-off). This approach tends to be costly in practice, particularly for ballasts that power multiple lamps. Accordingly, there is a need for improved programmed start ballasts.
Step-dimming ballasts have been developed to allow energy savings by users selecting one of two levels of fluorescent lamp illumination. Step dimming has been previously accomplished by dedicated dimming circuitry that increases the frequency of the ballast inverter to lower the output power, or by the provision of multiple inverters in the ballast, with one inverter being shut down while the other keeps working for dimmed operation. These dimming solutions, however, require additional circuit components and can be costly in terms of circuit area and cost. Thus, there is a need for improved step dimming ballasts.
Another problem with lamp ballasts relates to arcing. Electronic ballasts are generally equipped to provide high output voltages in order to ignite gas discharge lamps. As a result, however, these ballasts may be exposed output arcing fault conditions, such as when a failed lamp is while AC power is applied to the ballast, or when the lamp electrical connection with the ballast output is intermittent. Such arcing is undesirable and may damage the ballast and/or the lamp, lamp holder. Thus, it is desirable to provide improved electronic ballasts that can quickly extinguish detected arcs without damage to the ballast or the lamp holder.